1. Technical Field
The invention relates to the enhancement of the response of semiconductor imager devices, and specifically silicon charge coupled device (CCD) imaging chips, to ultraviolet and blue light.
2. Background Art
The intensive commercial development of imaging CCDs has strongly advanced the technology in the last several years, giving high yields, large area detectors, higher pixel counts, substantial increases in well depth, and a major reduction in dark current. Silicon-based scientific CCDs (of the type described in Janesick et al., "History and Advancements of Large Area Array Scientific CCD Images," Astronomical Society of Pacific Conference Series, 1991, Tucson, Ariz.) have also matured during this period, providing the astronomy community access to the unique characteristics of this device, which include larger pixel-to-pixel dynamic range (&gt;10000:1), quantum efficiencies in the X-ray and visible wavelengths approaching 80%, and broad band spectral response from 10 .ANG. to nearly 1 micron. Unfortunately, the absolute quantum efficiency (QE) of an untreated scientific CCD drops below 10.sup.-5 in the critically important wavelength region of 1000 .ANG. to 3000 .ANG.. This QE reduction is related to two primary issues: (i) the depth of photon absorption and electron-hole pair production in silicon at these wavelengths and (ii) the surface charge and unstable response inherent in an untreated backside-illuminated CCD. Dramatic improvements in performance have been demonstrated by modifying the CCD surface. Unfortunately, as will be discussed below, most of these improvements are unstable. What is needed is a modification providing the maximum ultraviolet quantum efficiency permitted by reflection limitations with long term device stability sufficient for space applications.
FIG. 1 illustrates a qualitative energy band diagram across the thickness of a backside-illuminated CCD imager substrate, and indicates the presence of a potential well at the back surface. The absorption depth problem is illustrated in FIG. 2, which gives the photon absorption depth versus incoming radiation wavelength for crystalline silicon. Note that the absorption depth drops to a minimum of 40 .ANG. at a wavelength of about 2800 .ANG. and is less than 600 .ANG. over the range of wavelengths from 600 to 4000 .ANG.. This short absorption depth means that electron-hole pair production is occurring within 500 .ANG. of the silicon surface. Because of the charge in the native oxide of the CCD backside surface, these carriers are swept into the backside surface potential well of FIG. 1 and undergo recombination leading to carrier annihilation, so that they never reach the photosensitive elements of the CCD on the opposite face of the silicon chip. This, of course, greatly reduces the quantum efficiency of the device.
The relationship between ultraviolet quantum efficiency and the surface potential well can be understood by reference to the qualitative energy band diagram of FIG. 1. The CCD charge collection wells and transfer circuits are located on the right side of the diagram. In this device, the gate oxide, gate metal, interface defect density, and ion implanted channel structure are all carefully optimized using the comprehensive technology of silicon metal-oxide-semiconductor (MOS) processing. The device substrate has been chemically thinned to give an imaging structure which is less than 20 micrometers thick. Thinning of the CCD substrate or die is done chemically.
In the standard fabrication of a backside thinned CCD, a native oxide is permitted to form through long term exposure to the atmosphere. This oxide is nonuniform in thickness, composition and defect density, and generally exhibits a substantial positive fixed oxide charge. This charge leads to a bending of the conduction and valence bands near the surface, as illustrated on the left side of FIG. 2. This band bending gives rise to a potential well (generally termed the backside well) which traps electrons generated within the well. This potential well prevents the detection of photons absorbed within approximately 1000 .ANG. of the backside surface. The thin native oxide is full of interface states or localized traps (with surface densities of 10.sup.11 to 10.sup.12 cm.sup.-2) which lead to electron-hole recombination (carrier annihilation). Efforts to produce better oxide layers with stable and controlled charge densities on the CCDs are limited by the susceptibility of the CCD circuitry to destruction during high temperature processing.
The overall problem becomes apparent by comparing the absorption depth in crystalline silicon of ultraviolet photons (illustrated in FIG. 2 as being about 40 .ANG. at a wavelength of 2800 .ANG.) with the vertical extent (about 1000 .ANG.) of the backside surface potential well of FIG. 1. According to FIG. 2, a large fraction of incident UV photons are absorbed within 100 .ANG. of the surface, producing carriers within the backside potential well, while according to FIG. 1 virtually all such carriers are absorbed into the backside surface potential well, never to reach the CCD detector elements on the front side, leading to a quantum efficiency of virtually zero for UV photons.
A number of solutions have been demonstrated to alter the effect of the oxide charge, all of which are perturbations to the completed thinned device structure, as described in Janesick et al., "History and Advancements of Large Area Array Scientific CCD Imagers," Astronomical Society of Pacific Conference Series, 1991, Tucson, Ariz., and Janesick et al., "Charge Coupled Device Pinning Technologies," SPIE Vol. 1071--Optical Sensors and Electronic Photography, 1989, pp. 153-169. These broadly include the introduction of negative charge at the native oxide surface, implantation of a narrow p+ layer on the CCD backside, and the use of a chromophore to convert UV photons to visible photons. The approaches relying on negative charge added to the surface of the backside oxide to create an accumulation layer include UV charging, chemical charging (nitrogen oxide adsorption), and biasing a thin metal layer (biased flash gate). Each of these engineering solutions afford dramatic enhancement of the short wavelength response, but also suffer from serious yield variations and/or pose potential long term reliability concerns. They are particularly compromised by open face operation of the CCD in modest vacuum and at reduced temperatures (on the order of -100.degree. C.). A quantum effiency of about 20% in the UV range is obtained by adding an organic molecular chromophore (phosphor) on the CCD. There remain some reliability, radiation damage and compatibility issues for long term space applications of these devices.
A commercially available solution is the introduction via ion implantation of a p+ layer at the silicon surface to bend the semiconductor energy bands upward and screen the effects of the fixed charge in the native oxide. This approach is the method of choice for detectors which are already in production for commercial CCD vendors such as Tektronix and Thomson CSF. The backside well, though reduced in extent by the ion implanted dopants, is still present due to the difficulty of getting a sharp doping profile by implanting and annealing the surface. Furthermore, damage caused by ion implantation reduces the quantum efficiency even after annealing.
The limited improvements achieved by the foregoing ion implantation techniques are compared in FIG. 3, illustrating the calculated spatial dependence of the conduction band edge near the backside surface of a CCD resulting from the various techniques. The curve for 200 .ANG. of 5.times.10.sup.18 B/cm.sup.3 corresponds to the current state-of-the-art for ion implantation technology. The curve for 100 .ANG. of 5.times.10.sup.19 B/cm.sup.3 corresponds to the ideal best that ion implantation with annealing could ultimately accomplish, in which case the vertical extent of the potential well is reduced to 50 .ANG.--still greater than the absorption depth of UV photons, unfortunately. The curve for 50 .ANG. of 3.times.10.sup.20 B/cm.sup.3 represents a typical dopant level for a MBE growth of a uniformly doped layer on the backside surface of the CCD. This latter technique, conceived by us, narrows the surface potential well to about 30 .ANG. (as shown in FIG. 3), which is about half the absorption depth of UV photons, a significant improvement but not necessarily a perfect solution.
Thus, there has appeared to be no way in which to generally prevent the annihilation of a significant fraction of UV-generated electrons at the backside surface.